Ketones with an ethylenic unsatuation alpha beta to the carbonyl group have a number of uses. The utility of these monomers is based on the activation of the carbonyl group by ultraviolet radiation.
Such ketones may be used in photoresist applications such as making microcircuits. For example methyl isopropenyl ketone (3-methyl-3-butene-2-one) or MIPK is useful in photoresist applications as disclosed in:
CA 98:82425c of Japanese Kokai 57,159,045; PA1 CA 100:219136r of Japanese Kokai 58,39,779., and PA1 CA 101:46307n of Japanese Kokai 58,52,634. PA1 one or more monomers of the formula ##STR2## wherein R.sub.1 is a hydrogen atom or a C.sub.1-4 alkyl radical; PA1 and PA1 dimers thereof; PA1 (i) benzoyl peroxide; and PA1 (ii) peroxides of the formula ##STR3## wherein R.sub.4 is a C.sub.1-6 branched or straight chain alkyl radical or a C.sub.6-10 aromatic radical and n is 1 or 2, at a temperature and for a period of time so that the weight percent of a residual dimer in said mixture is reduced by at least 20 percent.
Such ketone monomers are also useful in the manufacture of photodegradable plastics as disclosed in Canadian Patent 1,000,000 granted Nov. 16, 1976 to James E. Guillet and Harvey Troth.
Specific ketone monomers are useful in other applications. For example MIPK may be used in the production of isoprene as disclosed in U.S. Pat. No. 3,009,005 issued Nov. 14, 1961.
Unforunately, these ketones tend to form dimers. It is believed that the monomers undergo cyclodimerization. For example MIPK would be expected to form a 2,3-dihydropyran derivative. These dimers are an impurity and should be removed. Furthermore, in cases where the ketone is subjected to free radical polymerization, the dimers act as inhibitors. While it is possible to overcome the inhibiting effect of the dimer by using additional initiator this results in a lower molecular weight polymer which is undesirable and also more expensive.
U.S. Pat. No. 2,851,497 teaches that such dimers may be converted to the monomer by treating the dimer in a vapour phase at a temperature of at least 375.degree., preferably 420.degree. to 520.degree. C. While this may be an effective way of reducing or removing dimer, it is energy and capital intensive, and possibly dangerous.
The present invention seeks to provide a simple method to "reduce" such dimers. In accordance with the invention the dimer is not physically removed from the monomer but is believed to be converted to a different innocuous species. The decrease in dimer content may be monitored using conventional analytical procedures, such as chromatography.